Nano round polycrystalline adsorbent of chicken bones origin for Congo red dye adsorption

Nano round polycrystalline adsorbent (NRPA) of chicken bones origin was utilize as effective adsorbent in Congo red dye removal via aqueous media. The NRPA adsorbent was prepared via thermal decomposition and its structure was investigated with the aids of Transmission Electron Microscopy, Fourier Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (EDX), and X-ray Diffractometer (XRD). A monophasic apatite phase was confirmed from XRD investigation, while functional groups analysis showed that NRPA possessed CO32−, PO43− and OH− absorption bands. The maximum adsorption capacities derived from Langmuir isotherm is 98.216 mg g−1. From the combined values of n from Freundlich and separation factor (RL) of Langmuir models, the adsorption of CR by NRPA is favourable. Thermodynamic values of 5.280 kJ mol−1 and 16.403 kJ mol−1 K−1 were found for ΔH° and ΔS° respectively. The entire values of ΔG° which ranges from − 35.248 to − 459.68 kJ mol−1 were all negative at different temperatures. Thus, nano polycrystalline adsorbent of chicken bone origin can serve as excellent adsorbent in Congo red dye removal from waste water.


Characterizations
TENSOR 27, series FT-IR spectrometer, (Germany) was deployed to examine the various binding groups available on the adsorbent surface using the KBr pellet method.Mixture of KBr powder and NRPA was done in a mortar and pestle using a ratio of 1:99% and thereafter compressed to form a 2 mm pellet.An X-pert PRO, PANalytical, (Netherland) diffractometer was deployed for the phase and purity assessment of the NRPA powder (wavelength = CuKα1).A Carl Zeiss AG (Supra 55VP) scanning electron microscope was deployed for the adsorbent surface morphology analysis, while chemical configuration of NRPA was done using Energy-dispersive X-ray spectroscopy (EDX).Further imaging was achieved with transmission electron microscope (Tecnai 20 G2 FEI, Netherland) after sonicating dispersed particles of the adsorbent in distilled water.Zero point charge (pH ZPC ) of NRPA was done with a Zetasizer Nano ZS instrument (Malvern, UK).A Quantachrome NOVA 2200C, (USA) analyzer was utilized to measured pore size, pore volume and surface area of NRPA.

Batch adsorption process
A magnetic orbital shaker was used to conduct the experiments with the content placed inside 100 mL conical flask.The experiment was done in triplicate and an average value was obtained.Efects of CR concentration was done between 25 and 200 mg L −1 , NRPA dose of 10 to 60 mg, time of 10 to 160 min and solution pH of 2.0 to 7.0.Briefly, 40 mg of the NRPA was added to 25 mL volume of CR solution.The solution was adjusted to desired value by adding either 0.1 M of NaOH or HCl, after which the content was equilibrated on the orbital shaker.At diverse time intervals, certain amount of CR was withdrawn and analyzed with the aids of Ultraviolet-Visible spectrophotometer at 498 nm wavelength.The equilibrium amount of CR adsorbed by the synthesized NRPA was evaluated using the relationship below: (1) Given that C o and C e in mg L −1 stand for CR initial and equilibrium concentrations, V represent solution volume in L, while m denotes NRPA amount used in mg.Percentage removal (R%) of CR was obtained thus: The experiments were performed in triplicate and the average values was reported.

Desorption study
To assess the reusability of NRPA, adsorption cum desorption experiments were performed in six cycles using the already utilized adsorbent.Briefly, the desorption experiments were done by adding 15 mL solution of 0.1 M HCl to already used NRPA and placed on orbital shaker for agitation for 60 min.The content was filtered, and the CR concentration left in the filtrate was measured using spectrophotometer.The NRPA was used for another five cycles after each adsorption after thoroughly washing it with distilled water.

Adsorption isotherms
Initial and equilibrium CR concentration relationship was examined using four common adsorption isotherms of two-parameters (Dubinin-Radushkevich (D-R), Langmuir, Temkin, and Freundlich), and three-parameters isotherms (Sips and Redlich-Peterson (R-P)).A Microsoft Math Scientist version 3.0.was deployed to predict the acceptability, while the suitability of the models was estimated from the values of the correlation coefficients, R 2 .The Langmuir, D-R, Temkin, and Freundlich isotherm are listed in Eqs.(3-6) 1-4,19-25 : Langmuir isotherm assumed that the entire surface where the adsorption process takes place is uniform and no interaction between the adsorbed molecules 20 .Q max denotes maximum uptake of CR (mg g −1 ), C e (mg g −1 ) denotes CR concentration at equilibrium and b represents constant of Langmuir in L mg −1 .
Separation factor (R L ) from the Langmuir model is given as 23 : Isotherm of the Freundlich assumed a heterogeneous surface in which adsorbed molecules interact together 1,2,6 .The Freundlich constants of n and K F stand for adsorbent intensity and adsorption capacity respectively 1,2,6 .Temkin isotherm assumption is such that the surface coverage of the adsorption is a function of the free energy 23 .The parameters 'a T ' and 'b T ' ' denote binding energy in L mg −1 and adsorption heat process respectively, T stands for temperature (K), while R denotes the ideal gas constant (8.314J mol −1 K −1 ).D-R isotherm can be utilized for the classification of adsorption processes into either physisorption or chemisorption 23 .Saturation capacity in mol g −1 is given as Q s , while Polanyi potential (ε) is expressed as: The expression β in mol 2 J −2 denotes the adsorption process mean free energy (E) (kJ mol − ) 19,23 .The mathematical expression is given as: When E is lesser than 8 kJ mol −1 , it is physisorption, but when E is between 8 and 16 kJ mol −1 , it denotes chemisorption 19 .
The three-parameters isotherm models of R-P and Sips are represented as 26,27 : where K R, in L mg −1 stands for the R-P constant; g denotes the R-P exponent that vary between 0 and 1. Langmuir isotherm equation is predicted if β is 1 and when β equals 0, the R-P reduces to Freundlich isotherm equation 24,25 .Ks represents the Sips isotherm equilibrium constant in L mg −1 , Q s represents adsorption capacity in mgg −1 . (2)

Adsorption Kinetics studies
Mechanisms of adsorption involved in the uptake process was further explored by Intra-particle diffusion kinetic, pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich models as listed in Eqs.(12-15) below [1][2][3][4]19 : The rate constant relating to intraparticle diffusion in mg g −1 min −0.5 ) is given as K id , while the intercept which defines the adsorbent surface thickness is given as C i .Given that Q e and Q t stand for the pollutant concentrations adsorbed in mg g −1 at equilibrium and time t (min), k 1 and k 2 represent adsorption rate constants in min −1 and gmg −1 min −1 for the first-and second-order respectively, t denotes contact time (min), α and β are the Elovich constants representing the initial adsorption rate (mg g −1 min −1 ) as well as the desorption constant (g mg −1 ) respectively.

Statistical test
Least square fit was the basis for best fit, it becomes imperative therefore to compare distribution error for the acceptability of the best model fit.For this purpose, the root mean square error (RMSE) and sum square error function (SSE) were engaged to confirm the best fit kinetic and isotherms models fit as indicated in Eqs.(20, 21) 24,25 : The parameters numbers in the model and the number of data points are given as P and N respectively.The lower the values of the distribution errors, the more acceptable the model.

Thermodynamics studies
The spreading of the molecules of the solutes in solution at equilibrium can be deploy to determine constant of equilibrium, K d , expressed as 19,27,28 : The free energy change (∆G°), enthalpy change (∆H°) and entropy change (∆S°) can be deduced from the equilibrium constant in the relationships below 1-3 :

Characterization of NRPA powder
The chicken derived nano-polycrystaline adsorbent powders were examined using FT-IR before and after CR adsorption as represented in Fig. 1.The NRPA powder showed major stretching vibrations between 1025 and 1055 cm −1 indicating the presence of phosphate 19 , O-P-O bending (v4) were noticed at 567 and 610 cm −1 .The bands appearing at 887 and 1465 cm -1 designate the presence of carbonate group.The O-H stretch was noticed between 3365 and 3664 cm −1 .After the adsorption of CR, the wavelength of the NRPA shifted with new peaks appearing in the ranges of 1023-1045 cm −1 , 885-1462 cm −1 and 3360-3655 cm −1 indicates that phosphate, (12) carbonate, and hydroxyl functional groups were involved in the adsorption process.These shifts in bands position is a confirmation of NRPA interaction with the dye molecules.Figure 2 represents the XRD patterns of the NRPA adsorbent before and after the uptake of CR molecules.As depicted, the peaks of the NRPA powder were identified by JCPDS file no.090-432 and corresponding main planes of hydroxyapatite were confirmed at (002) and ( 211) planes.The poly-round crystalline nature of NRPA was confirmed by the occurance of many sharp peaks.Oher standard peaks assigned to hydroxyapatite powder were equally detected.Reductions in peaks, peaks broadening coupled with reduction in peaks intensity were observed after the uptake of CR which further confirmed the interaction between the NRPA adsorbent and the CR molecules.SEM image of chicken bone derived adsorbent is shown in Fig. 3a with morphology of round shapes and particle size of between 10 and 44 nm.The round shape morphology was equally confirmed by the TEM analysis (Fig. 3b), while the selected area electron diffraction (SAED) also corroborated the polycrystalline nature of the apatite with spotted sharp and continuous rings system (Fig. 3c).The EDX revealed that oxygen, phosphorus and calcium are the major composition of the chicken bone with some trace amount of elements such as carbon, silicon, magnesium and potassium thus indicating the biogenic nature of the adsorbent (Fig. 3d).

Contact time and CR initial concentrations
The maximum uptake of CR as it relates to contact time is depicted in Fig. 5.As seen in the plot, CR dye removal rose progressively with the contact time pending the attainment of equilibrium point at 100 min.An increase in capacity of adsorption was noticed from 6.35 to 10.27 mg g −1 on altering contact time from 10 to 100 min at 25 mg L −1 CR concentration and from 26.33 to 103.47 mg g −1 at 200 mg L −1 .After attaining the highest adsorption ability at 100 min, the CR removal was noticed to uniformed with time.The initial enhancement of the uptake rate of the dye can be attributed to the accessibility of excess adsorption sites of the adsorbent which gradually becomes occupied as the uptake time increases and as such, the adsorbent ability gradually became exhausted and only few vacant sites remained.Similar observation was documented by Wanyoni et al. 29 for Congo red dye removal.Figure 4 showed that CR concentration equally affects the uptake of the contaminant.The uptake potency of NRPA adsorbent was enhanced when CR concentration was increased.Adsorbate concentration increase enhanced mass gradient transfer of CR molecules in aqueous phase and the solid phase which is the driving force for the transportation of CR particles from aqueous solution onto NRPA surface 30 .
The role of NRPA dose Figure 5 reveals the role of NRPA dosage on the consumption of CR at two diverse temperatures of 298 K and 318 K respectively.The uptake efficacy increased from 53.44 to 78.65% at 298 K and from 66.34 to 95.30% at 318 K when the NRPA dosage was raised from 10 to 40 mg respectively.This phenomenon can be rationalized on the accessibility of much sites as a result of large surface area which enhanced the uptake process 19,29 .But, www.nature.com/scientificreports/further rise in the dosage of NRPA resulted in lower uptake of CR owing to aggregation of CR particles on the adsorption sites which reduces the available sites for the uptake of the CR dye 19,29 .

Effect of Congo red dye pH
The role that pH plays on CR adsorption was explored in temperature range of 298 K to 338 K (Fig. 6).Maximum adsorption percentage removal of CR at 298 K, 308 K, 318 and 338 K were recorded at maximum pH of 2 to be 77.40%,87.40%, 94.70% and 70.33% respectively.Solution pH is very vital in sorption process owing to the fact that the chemistry of both the adsorbent as well as the adsorbate medium can be greatly affected.The pH ZPC of NRPA was deduced to be 3.5 and below this, NRPA surface is anticipated to be positively charged.Congo red being an anionic dye is expected to exist as negative charge in acidic pH and this enhances CR adsorption onto the positively charged NRPA surface 2,7,31 .Congo red is zwitterion molecular and an acid-base indicator as a result of the sulfonated group (-SO 3 − ) and amine group (-NH 3 + ).At strong acid condition (pKa < 5), Congo red colour turns blue and the concentration of H = in the solution is high and when the adsorbent surface at lower pH adsorbed H + , it becomes positively charge which enhances the adsorption of the negatively charged sulfonated group of the Congo red via electrostatic attraction, which enhances greater performance of the adsorbent.However, with increase in pH value above the pH ZPC , deprotonation of carboxyl, phosphate and hydroxyl functional groups available on the NRPA surface occurs which led to a repulsive attraction amid the sites of the adsorbent which are negatively charged as well as the anionic dye.According to Ogundiran et al. 30 , optimum pH of 2.0 was reported for CR uptake by unmodified snail shell and acid modified snail shell with maximum percentage removal of 65% and 77% respectively.It was concluded that the electrostatic interaction between the adsorbent surface which is positively charged at lower pH and various functional groups of CR molecules which are negatively charged were responsible for the high uptake of CR at lower pH and that at higher pH, the adsorption of the anionic dye decreases due to deprotonation on the surface of the biomass 30 .www.nature.com/scientificreports/Kumar et al. 32 also observed that the reaction of molecules of CR with polyaniline@MoS 2 -based organic-inorganic nanohybrid surface decreases with rise in solution pH, which was due to the electrostatic relationship between protonated Pani@MoS 2 and the negatively charged molecules of CR which took place in the acidic medium, thus enhancing CR molecules adsorption.Similarly, Munagapati and Kim 33 reported that during the uptake of molecules of CR by CABI nano-goethite adsorbent, it was observed that the maximum sorption capacity of CABI nano-goethite was 137.7 mg g −1 at pH 3.0.They opined further that at this pH, the amount of hydrogen ions in the medium is much and that the adsorbent surface gains positive charge which resulted in the high uptake noticed at pH 3.0.Vairavel et al. 34 observed that the sorption percentage of CR reduced from 82.35 to 11.41% when pH rose from 6 to 12 and that the negatively charged CR molecules is as a results of excess hydroxyl ions in the medium which resulted in greater electrostatic repulsion amid the adsorbent surface and the molecules of dye at higher solution pH.

Isotherms studies
Figures 7 and 8 were used to deduce the constants of Freundlich, Temkin, D-R, and Langmuir as well as R-P and Sips isotherms respectively, while their constants are provided in Table 1.Correlation coefficient (R 2 ) deduced are 0.996, 0.987, 0.977, 0.994, 0.984 and 0.986 for Langmuir, Freundlich, Temkin, D-R, Sips and R-P isotherms respectively and on the strength of the R 2 , the root mean square error (RMSE) as well as the sum square error function (SSE) values, the Langmuir isotherm performed better and thus governed the adsorption data.Therefore, relying on Langmuir assumptions, CR dye particles binding onto NRPA surface is monolayer adsorption with the receptor sites occurring on a homogeneous surface 30 .From the combined values of n from Freundlich and separation factor (R L ) from Langmuir models, the adsorption of CR by NRPA is favourable 1,2,7 .The maximum adsorption capacities derived from Freundlich, Langmuir, Temkin, D-R, Sips and R-P isotherms are 44.562mg g −1 mg L −1/2 , 98.216 mg g −1 , 26.327 mg g −1 , 79.355 mg g −1 , 62.103 mg g −1 and 49.205 mg g −1 .Value of E obtained from D-R isotherm is 0.595 kJ mol −1 and this is smaller than 8 kJ mol −1 and as such, CR adsorption onto NRPA surface is said to be physical in nature 1 .The adsorption data were further examined using three parameters' isotherms of R-P and Sips and the results obtained revealed good fit with both isotherms which combines the features of Freundlich and Langmuir models.When the adsorption capacity of NRPA was adjudged with further adsorbents in literature as indicated in Table 4, NRPA demonstrated higher adsorption affinity towards CR.The results further showcase chicken bone derived nano round polycrystalline adsorbent as a hopeful adsorbent in the elimination of CR from contaminated water.www.nature.com/scientificreports/adsorption is said to be physical process when ΔH° < 25 kJ mol −1 but chemical when ΔH° > 40 kJ mol −1 .Value of ΔS° obtained is 16.403 kJ mol −1 K −1 which indicates a rise in the gradation of haphazardness at NRPA/CR boundary of the adsorption process 37 .Numerous mechanisms of adsorption are available to further elucidate the interactions which exist between the adsorbate (CR) and the adsorbent (NRPA) among which are ion-exchange, chemisorption, precipitation, complexation, electrostatic attraction, and physisorption 23 .The FT-IR study of NRPA showed the presence of CO 3 2-, O-H and PO 4 3− which could become positively charged when the solution of CR is protonated at acidic pH and thereafter reacts with negatively charged CR through strong electrostatic attraction mechanism.This was corroborated further from the value of the pH ZPC of NRPA which was obtained to be 3.5 and as such, NRPA surface is anticipated to be positively charged below this pH ZPC value.Thus, Congo red being an anionic dye in acidic pH medium can easily adhered to the positively charged NRPA surface.According to Zhongxin et al. 45 , the sorption of Congo red by orange peel biochar modified with CTAB is electrostatic attraction and this is caused by the positively charged mango peel biochar modified with CTAB and Congo red which negatively charge is responsible for the adsorption.Maria et al. 19 opined that adsorption mechanism of Congo Red dye by fly ash can be described through strong electrostatic attraction between fly ash surface which is mainly positive and CR molecules which is negative charged.They stressed further that functional groups such as -OH and Si-O-Si present on the adsorbent surface were responsible for the formation of electrostatic attractions with CR molecules.Using the values of correlation coefficients and the predicted values for qe and the experimental ones, Khaoula et al. 46 proved that the adsorption of CR by pine bark is controlled by chemisorption mechanism.Their finding revealed that the predominant functional groups presence on the surface of pine bark at acid pH are positive, whereas, since the isoelectric point of Congo red is 3, the molecular structures is expected to be negative when the pH is greater than 3 which subsequently resulted in electrostatic attraction mechanism.Figure 11 shows the schematic representation of adsorption mechanism of CR by NRPA.Luffa cylindrica cellulosic fiber 17.39 Gupta et al. 42 Cationic surfactant modified wheat straw 71.2 Cheng et al. 43 Pani@MoS 2 70.920Kumar et al. 32 PVC@GN-Pani-NH4OH 26.315 Kumar et al. 44 PVC@GN-Pani-HC 31.250Kumar et al. 44 PVC@GN-Pani-CTAB 40.000Kumar et al. 44

Desorption study
The results obtained from desorption experiments with HCl as eluting agent is shown in Fig. 12.It was revealed from the results that 0.1 M solution of HCl clearly desorbed the adsorbate from the surface of the adsorbent after each successive experiment.It was observed that the NRPA adsorption-desorption efficiency for CR was almost constant for the first three cycles with desorption efficiency of 83.88% after the fourth cycle.However, above the fourth cycles, the percentage adsorption decreased sharply indicating reduction in the adsorption ability of the adsorbent.The results clearly shows that the NRPA can be use more than one time and thus, demonstrate it's regeneration and economic importance.

Conclusion
This work investigates the adsorption potency of chicken bone derive adsorbent for anionic Congo red dye removal.FT-IR, XRD, TEM, SEM, and pH pzc analyzes were done to characterized the fabricated adsorbent.Raw data obtained were subjected to different adsorption isotherms with the Langmuir model showing better conformity.Optimum adsorption was attained at solution pH of 2.0, adsorbent dosage of 40 mg, contact time of 100 min and initial CR concentration of 200 mg L −1 respectively.Data from kinetics examinations showed that the uptake of CR by NRPA obeys PFO model.The maximum adsorption capacities derived from Langmuir isotherm is 98.216 mg g −1 .Constants of ΔG° calculated ranges from − 359.248 to − 4459.68 kJ mol −1 were all negative at different temperatures tested thereby suggesting that the uptake of CR by NRPA surface is spontaneous process and endothermic nature.On the strength of the above, chicken bone derived adsorbent was found to be a suitable adsorbent in the uptake of anionic Congo red dyes from contaminated water.

Figure 1 .
Figure 1.FT-IR analysis of the nano round polycrystalline adsorbent (NRPA) obtained from chicken bone before and after adsorption.

Figure 2 .
Figure 2. XRD analysis of the nano round polycrystalline adsorbent (NRPA) obtained from chicken bone before and after adsorption of CR.

Figure 4 .
Figure 4. Function of contact time and CR concentration for the uptake of CR dye by nano round polycrystalline adsorbent (NRPA).

Figure 5 .
Figure 5. Function of nano polycrystalline adsorbent dosage for the uptake of CR dye by nano round polycrystalline adsorbent (NRPA).

Figure 6 .Figure 7 .
Figure 6.Function of pH on the uptake of CR dye by nano round polycrystalline adsorbent (NRPA).

Figure 8 .
Figure 8. Three parameters isotherms for the adsorption of CR by nano round polycrystalline adsorbent (NRPA).

Figure 10 .
Figure 10.Plot of thermodynamic parameters for the adsorption of CR by nano round polycrystalline adsorbent (NRPA).

Figure 11 .
Figure 11.Schematic representation of adsorption mechanism of CR by NRPA.

Table 1 .
Isotherm parameters of NRPA in the adsorption of CR.

Table 2 .
Kinetic parameters of NRPA in the adsorption of CR.

Table 3 .
Thermodynamic parameters of NRPA in the adsorption of CR.

Table 4 .
Adsorption Capacities of various Adsorbents with NRPA in the adsorption of CR.